It has been widely reported that whole blood oxygen affinity varies in different disease states characterized by anemia. The fact that whole blood oxygen affinity is reduced in persons with sickle cell anemia has been known for many years. A recent report indicates that non-transfused children with homozygous B thalassemia may have a paradoxical increase in anemia. There is very little quantitative information concerning serial changes in whole blood O2 affinity post-transfusion in those disease states which require transfusion of red cells. It is quite possible that post-transfusion changes in O2 affinity actually result in a more severe functional anemia despite the hemoglobin elevation. If the red cell milieu is important, as the report concerning thalassemia major suggests, whole blood O2 affinity post-transfusion may differ significantly in such different disease states as thalassemia, sickle cell anemia, aplastic anemia and myeloid metaplasia. In this study, whole blood O2 affinity will be measured in patients who require transfusion (1) prior to transfusion (2) daily for 5 days (3) weekly thereafter for three weeks. Whole blood oxygen affinity of the transfused blood will be measured as well. A quantitative estimate of the percent of transfused cells remaining in each sample will be derived by differential agglutination (Ashby technique).